Silicon nitride (Si.sub.3 N.sub.4) is useful in applications that require oxidation and wear resistance, high emissivity, and low thermal and electrical conductivity. Si.sub.3 N.sub.4 can be used in bulk form or as a matrix or coating for composites. For example, crystalline Si.sub.3 N.sub.4 can be used to coat carbon-carbon composites as taught in commonly assigned U.S. Pat. No. 4,472,476 to Veltri et al. Similar coatings and Si.sub.3 N.sub.4 processes are disclosed in U.S. Pat. No. 3,226,194 to Kuntz, and commonly assigned U.S. Pat. Nos. 4,214,037, 4,289,801, and 4,671,997 to Galasso et al. and 4,500,483 and 4,610,896 to Veltri et al.
Crystalline Si.sub.3 N.sub.4 can be made with a chemical vapor deposition (CVD) process described in U.S. Pat. No. 3,226,194 to Kuntz, herein incorporated by reference in its entirety. When used to make matrices for composites, the same process is called chemical vapor infiltration (CVI). Si.sub.3 N.sub.4 made by a CVD reaction often is called CVD Si.sub.4 N.sub.4.
The CVD reaction typically takes place at pressures less than about 40 kPa (absolute) inside a closed, cylindrical, graphite reactor. Substrates to be coated or infiltrated with matrix are held in place inside the reactor with graphite fixtures. After heating the substrate and interior of the reactor to a reaction temperature between about 1200.degree. C. and about 1700.degree. C., reactant gases are admitted to the bottom of the reactor. Typical reactant gases include silicon fluoride (SiF.sub.4) and ammonia (NH.sub.3). The reactant gases flow upwards (axially) through the reactor and react to form Si.sub.3 N.sub.4 until one of the reactant gases, usually SiF.sub.4, is expended. In prior art methods, the Si.sub.3 N.sub.4 deposits uniformly on the interior walls of the reactor, the fixtures, and the substrate. The radially uniform deposition has been attributed to homogenous mixing of the reactant gases within a few centimeters of the bottom of the reactor. Useable Si.sub.3 N.sub.4 is that which forms on the substrate. Because only a fraction of the reactant gases makes useable Si.sub.3 N.sub.4, the prior art method can be costly.
Another drawback of the prior art method is short reactor life. The combination of high reaction temperature, corrosive reactant gases (especially NH.sub.3), and temperature cycling can cause the graphite CVD reactors to crack after only several runs. Typical reactor life can be about 4 to about 12 runs. To protect expensive graphite heating elements that surround the reactor from corrosive reactant gases that may leak through cracks in the reactor, cracked reactors must be discarded. The need to replace reactors frequently also contributes to the costliness of the prior art method.
Therefore, what is needed in the industry is a method of making Si.sub.3 N.sub.4 that deposits less Si.sub.3 N.sub.4 on reactor walls and fixtures than prior art methods. It also would be desirable if reactor life could be extended.